The invention relates generally to interconnection and encapsulation of electronic components, in particular to interconnection and encapsulation methods for flip-chip integrated circuits, and specifically to material selection for interconnection and encapsulation of flip-chip integrated circuits.
Thermosetting resin compositions such as epoxy resins have been used as semiconductor device encapsulants for over 25 years as noted by reference to U.S. Pat. No. 3,449,641, granted Jun. 10, 1969.
U.S. Pat. No. 3,791,027, Angelo et al., describes epoxy fluxes for soldering. Angelo et al. teach that the fluxes may be formulated to be removable from the solder situs or may be formulated through cross-linking after the soldering process to form a thermoset epoxy polymer which remains at the solder joint and reinforces the strength of the solder joint.
Anhydride-cured epoxy resin encapsulants used in flip-chip manufacturing methods that are applied after electrical interconnection are described in U.S. Pat. No. 4,999,699, granted Mar. 12, 1991, and U.S. Pat. No. 5,250,848, granted Oct. 5, 1993.
The flip-chip method of attaching integrated circuits to substrate boards involves a series of metal solder bumps on the integrated circuit which form metallurgical interconnections with the metal bond sites on the board substrate. The active side of the integrated circuit is flipped upside down in order to make contact between the bumps on the chip and the metal bond sites on the substrate. An organic soldering flux is used to remove metal oxides and promote wetting of the solder when the assembly is heated above the temperature of the solder. This process is referred to as reflow soldering. The purpose of the flux is to clean the surface of the metals. The solder, or lower melting alloy, may be the composition of the board bond pads, of the bumps on the chip or both depending on the materials selected. Similarly, the higher melting alloy may be present on either the bond pad or the bumps on the chip. This process is derived from the controlled, collapse, chip, connect (C4) method developed by IBM in the 1960""s.
The reflow soldering operation provides a gap of 0.025 mm to 0.17 mm between the chip and the substrate. Although this small standoff height significantly enhances the electrical performance of the mounted flip-chip, the residue from the flux is difficult to remove from the narrow gap. Thus, no-clean fluxes, in which flux residues are not removed from the board after reflow soldering, are the flux type of choice for most flip-chip applications. These no-clean fluxes may be dispensed onto the metal bond sites on the board prior to chip placement. These liquid no-clean fluxes are formulated to contain more than 94% solvent which evaporates during the reflow process and flux activators which sublime during the reflow step. Thus, minimal amounts of residue remains on the board after reflow. These liquid fluxes, however, have difficulty in holding the chip to the board prior to reflow. The high solvent content of the flux causes the small integrated circuit to skew and misalign before peak soldering temperatures are reached. An additional problem arises from the volatility of many solvents used in these fluxes which blow the chips out of alignment during reflow. Although tackifying agents can be added to overcome these problems, the no-clean, low-residue requirement of the flux dictates a high solvent content which leads to alignment problems during reflow.
In order to maintain alignment of the chip to the board prior to reflow soldering, a viscous tacky flux may be applied to the bumps on the chip. This method involves dispensing the flux onto a rotating disk or drum then applying a blade above the rotating drum. Thus, a desired thickness of flux on the drum can be achieved by adjusting the height of the blade. The integrated circuit, containing solder bumps, is then dipped into the flux on the drum to a set depth. Using this method a desired amount of tacky flux is applied to the surface of the bumps only. The chip is then aligned and placed onto the substrate so that the bumps, which contain tacky flux, make contact with the appropriate metal bond sites. The tacky flux is formulated to contain a higher solids content which aids in the adhesion of the chip to the substrate prior to reflow. The tacky flux acts as a temporary glue to hold the chip in proper alignment during placement of the assembly into the reflow oven. The tacky flux contains less solvent which prevents the phenomenon of blowing the chips off the board during reflow commonly seen using liquid fluxes. Since only a small amount of flux is applied to the bumps, minimal residue remains on the board after soldering.
The tacky fluxes commonly used are the solderpaste flux vehicles used in no-clean surface mount processes. Although the formulations of no-clean solderpaste flux vehicles vary, a typical composition contains 50% rosin, 40% solvent, 5-8% thickeners, and 2-5% flux activators such as organic acids and amines. The rosin, or a synthetic resin with similar characteristics, does not boil-off during the reflow profile and is necessary to act as a carrier for flux agents at peak soldering temperatures. The residue which remains after soldering is typically rosin or a similar resin with any remaining ingredients such as decomposed organic acids, amines, thickeners, or other organic constituents of the solderpaste. When these solderpaste flux vehicles are used to solder flip-chip devices using the described drum flux method they provide desirable properties such as rolling on the drum, forming thin films and leaving minimal residue.
The flip-chip assembly is formed by soldering the solder bumps of the integrated circuit to the appropriate metal bond sites of the organic substrate. The resulting flip-chip assembly has a gap between the integrated circuit and substrate. This gap is generally filled with an underfill encapsulant. The liquid underfill encapsulant is dispensed around the sides of the soldered flip-chip and allowed to flow under the assembly by capillary action. The purpose of the encapsulant is to relieve the thermomechanical stresses on the solder interconnections that are caused by the difference in thermal expansion coefficients between the silicon IC (CTE=2.5 ppm/xc2x0 C.) and the organic substrate (CTE=15-20 ppm/xc2x0 C.). Typical underfill encapsulants used in flip-chip assemblies are composed of epoxy resins, curing agents and inorganic fillers to yield a cross-linked thermosetting polymer when cured. The properties of the cured polymer, such as the CTE and elastic modulus, help relieve the thermomechanical stress on the solder joints during thermal cycling testing. Thermal cycling tests involve repeated exposure of the flip-chip assemblies to cycles of cold and hot environments. This repeated cycling induces thermal fatigue on the solder joints as the chip and organic substrate expand at different rates. A typical thermal cycle test involves repeated exposure of the flip-chip assembly to two different liquids at xe2x88x9255xc2x0 C. and +125xc2x0 C. with 10 minute dwell time at each temperature. Thus, the overall purpose of the underfill encapsulant is to enhance flip-chip assembly reliability by relieving the thermomechanical stress on the solder joints. Flip-chip assemblies on inorganic substrates, such as ceramic, do not generally use an underfill encapsulant as the CTE of ceramic closely matches that of the silicon IC.
Several process and material property characteristics dictate the material selection of the underfill encapsulant. First, the epoxy underfill encapsulant must flow quickly under the chip to achieve fast production cycle times. The viscosity, surface tension and particle size distributions can be optimized to achieve efficient flow under the chip during the encapsulation step. To further reduce the underfill time the substrate may be heated in order to reduce the viscosity of the uncured epoxy material. This heating significantly enhances the flow speed of the material. It is common to heat the surface of the substrate board to 70xc2x0 C. prior to dispense of the encapsulant in order to achieve this effect. Second, the epoxy underfill must cure quickly in order to achieve fast production cycle times. Typical underfill encapsulants are epoxy formulations designed to cure, i.e. form irreversible cross-linked structures, at temperatures above 150xc2x0 C. Finally, the epoxy underfill encapsulant must adhere strongly to both the chip and substrate during thermal cycling tests. If the epoxy pulls away, or delaminates, from either the chip or substrate surface, proper stress relief on the interconnects will not be achieved. The interface between the chip and the underfill is critical for proper thermal cycle reliability enhancement. It has been found that the interaction between the no-clean flux residue and the epoxy underfill encapsulant is critical to achieve maximum adhesion and proper flip-chip reliability enhancement.
As discussed, typical solderpaste flux compositions used as tacky fluxes for flip-chip contain rosin or a similar resin. After reflow soldering, a residue of rosin and other organic constituents of the flux remain on the substrate. Although these no-clean residues are benign to the assembly in terms of their corrosivity, these residues have been seen to adversely affect the adhesion of the epoxy underfill encapsulant. These rosin residues can be reheated and softened or even liquefied. Rosin softens at 55xc2x0 C. Since the underfill encapsulant is dispensed under the chip at temperatures of 70xc2x0 C., the epoxy underfill comes in contact with a liquid or softened residue. During cure, at temperatures at or above 150xc2x0 C., the epoxy is unable to properly adhere to the chip or substrate surface as the tacky flux residue is in a softened or liquefied state. The liquid or soft residue from the flux forms a barrier between the epoxy underfill and the surfaces of the chip and substrate. This may lead to early delamination from the chip surface poor adhesion of the underfill encapsulant.
This delamination of the encapsulant from the chip can be detected and measured using scanning acoustic microscopy (SAM). The SAM technique detects the presence of voids between the surface of the chip and the epoxy underfill. The SAM is used to first measure the total area of coverage then used to detect changes from this baseline value after thermal cycling tests.
In accordance with the present invention, there are provided tacky flux compositions for use in the soldering of flip-chip assemblies which contain in the most general terms 1.) an epoxy resin; 2.) a chemical cross-linking agent with fluxing properties; and 3.) a solvent. The tacky flux compositions of the present invention are also referred to herein as epoxy-based fluxes or epoxy-based fluxing agents. The compositions may be employed to solder flip-chip assemblies to substrates, such as organic substrates with high CTE values relative to the silicon IC, which require an underfill encapsulant to enhance reliability performance by reducing thermomechanical fatigue on the solder interconnects. The residue is present at minimum levels so as not to interfere with the underfill of the encapsulant, is designed to co-react with the underfill encapsulant and reduces delamination of the underfill from the substrate and chip during thermocycling reliability tests.
The present invention involves using an epoxy resin flux which does not interfere with solder melt and after the soldering step is partially cured to provide enhanced bonding at the interface of encapsulant and residue.
One aspect of the present invention relates to a protocol for selection of components for soldering flux for soldering flip-chip devices to circuit substrates, based on the ability of the solder flux components, when used in combination,
(1) to provide adequate flux activity to clean the surface metal oxides for a variety of solder alloy and metal bond compositions;
(2) to provide latency during the selected solder reflow profile; and
(3) to enhance bonding at the residue and underfill interface.
The flux compositions of the present invention are comprised of (1) an epoxy resin; (2) a cross-linking agent for such resin, which also functions as a fluxing agent during reflow soldering; (3) one or a combination of solvents; and (4) optionally a catalyst. These components are selected to form, in combination, a composition which forms a stable bond site for the underfill encapsulant.
Thermosetting polymers with flux properties are described in U.S. Pat. No. 5,376,403 (Capote) the teaching of which is incorporated herein by reference thereto. In Capote, the thermosetting polymers serve as adhesives when used in combination with metal powders to form conductive inks. Thermosetting compositions with inherent flux properties are described in U.S. Pat. No. 3,791,027 (Angelo) the teaching of which is incorporated herein by reference thereto. The thermosetting resins of Angelo protect and reinforce the solder joints.
The requirement of the present invention of polymer reaction latency is described in U.S. patent application Ser. No. 08/644,912, entitled Encapsulant with Fluxing Properties and Method of Use in Flip-Chip Surface Mount Soldering, the teaching of which is incorporated herein by reference thereto. Thermosetting compositions described in Ser. No. 08/644,912, which function as soldering fluxes in surface mount soldering find particular utility as the soldering flux of the present invention which forms a thermosetting residue after solder reflow.
Epoxy flux systems, wherein a catalyst is utilized to provide required latency for soldering, may be used, provided that the residue, after fluxing, is compatible with the encapsulant and adequate bonding occurs at the interface of encapsulant and residue during co-curing.
The benefit of using a thermosetting flux composition called for in the present invention resides in such fluxes"" compatibility with the underfill encapsulant. The method of selection of a (1) an epoxy resin, preferably an epoxy, (2) a cross-inking agent with fluxing properties, (3) one or more solvents and (4) optionally a catalyst, is based not only on the ability of the composition to serve as a flux and reduce metal oxides during soldering but also on the ability of the thermosetting resin residue to form a stable bond site for adhesion of the underfill encapsulant.
As described in U.S. patent application Ser. No. 08/644,912, Encapsulant with Fluxing Properties and Method of Use in Flip-Chip Surface Mount Soldering, the teaching of which is incorporated herein by reference thereto, it is critical that encapsulant compositions which have fluxing properties remain in a liquid form at elevated temperatures below the melting point of the alloy used in the soldering operation. This is a critical feature of the flux used in the present invention. Indeed, the properties of successful encapsulants with fluxing properties that are useful in fluxing underfill compositions are those required for the flux of the present invention. If a thermosetting flux composition reacts to its gel point, at which point the composition is restricted from liquid flow, before the melting point of the alloy is reached the gelled polymer will inhibit the desired wetting of the soldering of the alloys of the interconnection. The gel point of the thermosetting polymer is reached when sufficient cross-linking takes place between the resin and cross-linking agent molecules to inhibit the flow characteristics of the polymer composition.
Once the gel point is reached, the reaction is irreversible and said composition cannot be reheated and remelted by definition of a thermosetting polymer. Although this thermosetting characteristic is essential to the present invention, it is critical that this gel point is reached only after proper solder interconnections are formed. The thermosetting flux compositions are therefore designed to provide latency and not reach a gel point until after solder interconnections are formed.
Although flip-chip on board applications typically use the surface-mount reflow profile described in detail in U.S. patent application Ser. No. 08/644,912, the use of the present invention is not limited to surface mount reflow soldering. There are a number of flip-chip applications which do not call for the staggered heating profile necessary in surface mount reflow soldering. The critical aspect of latency of cure is specific to the temperature profile used in the soldering process, the melting point of the solder alloy, and the mass of the epoxy flux composition. One aspect and objective of the present invention is to describe a method for selecting epoxy compositions which function as soldering fluxes that provide increased adhesion with an underfill encapsulant.
In accordance with one embodiment the present invention, an epoxy flux composition is dispensed onto a drum fluxer comprised of a rotating disk and a blade in order to form a thin film of said flux. Preferably the epoxy-based fluxing agent is applied in the form of a thin film of a thickness less than 10/1000 inch. A flip-chip device is dipped into the film of flux in order to apply a layer of said flux to the solder bumps on the chip. The chip is removed from the drum then placed on a substrate so the solder bumps make contact with the proper metal bond pads on the circuit substrate. The assembly is heated beyond the melting point of the solder alloy used, preferably, but not limited to a surface mount reflow profile, in order to form metallurgical connections between the flip-chip and the circuit substrate. After soldering, the circuit substrate is reheated, typically to 70xc2x0 C. and an underfill encapsulant is dispensed between the chip and substrate. The flip-chip assembly is then heated, typically to 150-165xc2x0 C., in order to cure the encapsulant material. During the dispense and cure stages of the underfill encapsulant, the flux residue used in soldering the flip-chip to the circuit substrate forms a stable bond site for the adhesion of the encapsulant.
In the above mentioned process the epoxy flux is applied to the bumps on the chip only. As the bumps on the chip are typically 0.025 to 0.125 mm high, it is estimated that only from about 3 to about 5 micrograms of said flux are deposited per bump depending on the height of the bump. Since the cure kinetics of a thermosetting polymer are dependent on mass, it is important to consider the thin-film cure properties of said flux when used in the process described. Therefore, a thin film of material will reach a gel point at a much faster rate than a large mass of the same material under the same cure conditions. Although several compositions are described in prior art in which a thermosetting polymer exhibiting flux action is used to promote solder wetting, compositions must be selected on their ability to provide latency as a thin film when used in the chip dipping flux application process.
The selection of components to be used in combination to serve as a flux as well as a non-melting adhesion site for underfill encapsulation is based on the overall characteristics of the formulation to 1.) provide adequate flux activity to clean the surface metal oxides for the solder alloy and metal bond compositions; 2.) provide latency during the selected solder reflow profile; and 3.) to enhance bonding at the residue and underfill interface.
In U.S. Pat. No. 3,791,027 (Angelo) the teaching of which is incorporated herein by reference thereto, thermosetting polymers are described which function as flux agents. Specifically, chemicals containing amide, amino, carboxyl, imino, or mercaptan functional groups which contain active hydrogen, which functions to reduce surface metal oxides, and are also capable of xe2x80x9ccross-linkingxe2x80x9d when reacted with appropriate resin functionalities such as epoxide, isocyanate, etc. to form thermosetting residues. In the present invention, polymer precursors, such as acid anhydrides, carboxy terminated polybutadiene, or amines, which contain one or more of the above noted chemical functionalities, have been tested and identified to promote adequate solder wetting at 360xc2x0 F.-550xc2x0 F. using 63Sn/37Pb solder to copper.
In applications of flip-chip soldering, solder bumps on the chip or the metal bond sites on the pads may be composed of various metallurgies. A common flip-chip/substrate combination is 5Sn/95Pb (m.p.=301-304xc2x0 C.) bumps and 63Sn/37Pb (m.p.=183xc2x0 C.). Also, copper substrates often contain an organic solder preservative, commonly known as OSP, such as a benzimidazole, in order to protect the copper from oxidation. The OSP coated copper or the high lead alloys are more difficult to wet by 63Sn/37Pb solder alloy than copper and therefore require stronger flux agents in order to promote proper solder wetting.
In U.S. Pat. No. 5,376,403 (Capote), the teaching of which is incorporated herein by reference thereto examples of curing agents are described which function to reduce surface metal oxides on powders in electrically conductive ink applications. Specifically, a method is suggested to chemically protect an anhydride cross-linking agent by reacting said anhydride with an alcohol or polyol to form a mono-ester acid. More specifically, the reaction product from a combination of anhydride and a polyol is identified and selected as a suitable component to function as a metal reducing agent only when chemically triggered at high temperatures and react with an appropriate resin to adhere said ink to the substrate. Finally, the esterified product from the reaction of the anhydride with the polyol significantly improved the reduction of surface metal oxidation over the neat anhydride as evidenced by increased electrical conductivity of conductive inks made in this manner.
Thus, reactions of an anhydride with an alcohol or polyol are seen to increase the ability of the anhydride to reduce metal oxides, hence increasing the flux activity. The esterified anhydride contains more active hydrogen sites for reduction of surface oxides than the neat anhydride. Also, the esterified anhydride is said to be chemically protected which delays the reaction of said esterified anhydride cross-linking agent with the resin thereby preventing premature hardening of the polymer below the melt point of the solder alloy used. This delayed reaction is critical to thermosetting fluxes which are useful to form the soldering flux of the present invention which forms a thermosetting residue after solder reflow for flip chip applications as the small mass of flux used rapidly increases the curing process. Therefore, esterified anhydrides, i.e. the reaction products between anhydrides and alcohols or polyols, are the cross-linking agents of choice for flip-chip flux compositions which leave thermosetting residues.